1. FIELD OF THE INVENTION
The invention relates to an improved foundation pile and process for preparing pile foundations using the improved pile.
2. THE PRIOR ART
In cases where the foundation soil below the foundation level of a building consists of ground of inadequate bearing strength, loading can of course be transmitted by piles to stronger layers of earth available at greater depth. The piles used to transmit the loading to the stronger earth are either precast concrete piles or in-situ piles. Further subdivisions, depending upon the process used to prepare the piling, are drilled piles and driven piles.
In one known process for the preparation of pile foundations using precast concrete piles, precast concrete or reinforced concrete piles are driven into the ground by means of a monkey or tup or the like to the depth of the bearing layers; an equal volume of earth is displaced with the possible result, more particularly in soft clays, that previously positioned piles of the same foundation may move horizontally. Furthermore, driving a precast concrete pile of that type into the ground produces substantial vibrations which may endanger nearby buildings or previously driven piles. Additionally, it is difficult to ascertain the final condition of a pile which has been driven into the ground, since the pile may be damaged, for instance, by the driving forces; the corresponding damage is underground and cannot be checked. As a result, the pile can fracture without detection while being driven.
It is also known in the art to use driven piles made of on-site concrete for pile foundations. A jacket tube or form is driven into the ground and cleared internally and the concrete pile is prepared on-site in the tube; after the concrete is set, the jacket tube or form is removed. The advantage of pilings of this kind over ordinary driven piles is that a pile base can be prepared by reaming the ground below the form or jacket tube so that foundation loading can be increased. Disadvantages of this known process are that preparation on site is very tedious; also, it is often impossible to utilize pile foundations using driven form piles made of on-site concrete in soft ground since the ordinary or reinforced concrete which forms an individual pile receives inadequate transverse support from the soft ground, in which event the unset concrete may mix with the earth, thereby degrading uniform quality of the pile.
To obviate disadvantages associated with driven piles of the two kinds hereinbefore described, pile foundations can be prepared with the use of drilled piles, which as in the previous case are on-site piles whose sinking tubes are sunk by drilling rather than driving. This process can be used with minimal vibration and in places where space is restricted. In this known process, to assure that the pile has adequate bearing strength, the concrete is injected or vibrated into the surrounding earth while the jacket or form tubes are pulled. Drilled piles of that kind can be devised as end-bearing piles, the pile foot being thickened, more particularly in sticky ground. To that extent, the ground near the pile end is reamed out by appropriate facilities. Since the operation is free from vibration and since there is no question of an equal volume of earth being displaced, pile foundations of this kind are particularly useful in building gaps. The main disadvantage of this process is its high cost and relatively severe settling which arises because drilling causes some loosening of the bearing layer of earth. Also, the preparation of this kind of on-site pile calls for considerable skill and attention on the part of the person in charge of drilling the pile foundation. If undesirable seeping water is not observed in the drilling tube below the real watertable level, ground settlement may occur because of hydraulic ground pressures which may then damage adjacent buildings. That occurs often in practice. Another disadvantage of the known process is that, as always with on-site piles, the quality of the finished pile is difficult to ascertain since the concrete might mix with the earth around it.
According to an earlier suggestion of mine to obviate the disadvantages of the processes, using precast concrete piles, a hole adapted to the pile diameter is drilled in the ground as deeply as the region of bearing layers, the pile is then guided into the bore hole, whereafter the pile is rammed tight like a driven pile, producing an anchorage in the bearing layers of earth. In this known suggestion the advantages of a driven pile are combined with the advantages of drilled piles.
According to another development based upon my suggestion, the process is improved by the bore hole being drilled directly be means of the precast concrete pile which is devised for flushing the bottom end of the bore hole and which has a scrifice drilling bit at its bottom end. This feature helps considerably to simplify and cheapen the process; the only disadvantage is that the pile used for direct drilling must have additional steel reinforcement to accommodate drilling torques.
The disadvantages observed in the prior art are overcome by the instant invention as will be hereinafter set out.